1. Field of the Invention
The present invention relates to a lock-up control system for controlling the lock-up clutch of a so-called lock-up torque converter employed in an automatic transmission, and specifically to technologies for optimally increasing or decreasing the pressure differential between the apply and release pressures, each applied to the associated face of the lock-up clutch, often called a lock-up piston, in accordance with the desired automotive-transmission and power-train designer- or the desired transmission specialist-selected dynamic characteristic or transfer characteristic.
2. Description of the Prior Art
For example, this type of lock-up controller has been disclosed in Japanese Patent Provisional Publication No. 8-21526, assigned to the assignee of the present invention. In recent years, many cars are equipped with automatic transmissions with so-called lock-up torque converters which act to mechanically couple the engine crankshaft to the transmission output shaft. As is generally known, the lock-up clutch includes an apply chamber and a release chamber. The lock-up clutch is generally controllable by the pressure differential between the apply pressure in the apply chamber and the release pressure in the release chamber. Today many more cars employing automatic transmissions with so-called lock-up torque converters are designed so that the lock-up clutch is controllable by the pressure differential between the apply and release pressures to operate at either one of three operational zones, namely an open converter zone at which the lock-up clutch Is released, a slip lock-up zone at which the lock-up clutch is partially engaged, and a full lock-up zone at which the lock-up clutch is fully engaged, rather than two modes, namely an open converter zone and a completely lock-up zone. Usually, these zones are determined depending on at least two factors, namely a throttle opening of a throttle valve and a vehicle speed. In modern automotive vehicles employing automatic transmissions with lock-up torque converters, the lock-up clutch tends to be operated in the lock-up zone for example during constant-speed straight-ahead driving in which torque-increase and torque-fluctuation absorbing functions are unnecessary. In order to prevent engine stall and to absorb torque fluctuations, for example when the vehicle is rapidly decelerated from the vehicle coasting state with the lock-up clutch operated at the lock-up zone, the lock-up controller is generally designed to shift the operational mode of the lock-up clutch from the lock-up zone to the open converter zone by varying the previously-noted pressure differential. In this case, there is a great delay of response time from the lock-up zone (the clutch engagement position) to the open converter zone (the clutch release position), if the lock-up clutch is fully engaged. To avoid this or to quickly instantly release the lock-up clutch with a relatively small delay of response time, the previously-noted Japanese Patent Provisional Publication No. 8-21526 teaches the use of the minimum possible capacity (or degree) of engagement of the lock-up clutch which capacity corresponds to a boundary capacity (or a boundary engagement force of the lock-up clutch) between the full lock-up state and the partial lock-up state, when the vehicle is rapidly decelerated from the vehicle coasting state with the lock-up clutch operated at the lock-up zone. The aforementioned lock-up control techniques, however, have not met entirely satisfactory results. The prior art lock-up control system also suffers from the drawback of undesired shock of shifting from one of the lock-up position and the release position to the other.